1. Field of the Invention
This invention relates to deep water drilling, and more particularly to apparatus and proceses for maintaining hydrostatic head control during such drilling.
2. Prior Art
Present day drill ships and semi-submersible drilling rigs are designed to operate in water up to 6000 feet and actual wells already have been drilled in over 4000 feet of water. Because much of the untapped oil reserves lies in such deep water it is expected that deep water drilling shall continue to become more common.
Both for safety and environmental control it is necessary to use a blow-out preventer (BOP) stack. When drilling at these depths it is desirable and in fact is the common practice to position the BOP stack near or on the sea floor. An example of such a drilling arrangement is illustrated in Cameron Iron Works, Inc.'s brochure entitled "A Marine Drilling & Control Package By Cameron Iron Works." Examples of other marine riser assemblies can be seen in U.S. de Saint Palais, et al U.S. Pat. No. 4,058,137 issued Nov. 15, 1977 and entitled "Riser Pipe for Pivotally attached Structure Used to Extract Petroleum from Beneath a Body of Water;" U.S. Jones Patent U.S. Pat. No. 4,078,605 issued Mar. 14, 1978 and entitled "Riser Pipe String;" U.S. Osborne Patent U.S. Pat. No. 4,130,995 issued Dec. 26, 1978 and entitled "VMP Riser Horizontal Bearing;" and U.S. Rohde Patent U.S. Pat. No. 4,214,843 and entitled "Subsea Grout Distributer." Because of the high pressure, it is preferable that only small (21/2" to 41/2" diameter) choke and kill lines be used. Unfortunately, this creates problems in maintaining hydrostatic head during gas kicks. In these situations gas bubbles begin to rapidly elongate when they reach the small choke line and can quickly empty the choke line of all drilling mud resulting in loss of the hydrostatic head. This is also true when utilizing the marine riser for returns of gas to surface.